This invention relates to a measuring arrangement, in particular for performing optical spectroscopic measurements on particulate or liquid samples. The invention also relates to a measuring method for performing optical spectroscopy on particulate or liquid samples.
It is known from food technology that particulate samples (e.g., cereal grain, seeds or the like) may be subjected to optical spectroscopic measurements to obtain information regarding certain properties of samples, such as their chemical composition, water content, surface hardness, etc.
For example, reflection or transmission measurements may be performed with wavelengths in the visible or infrared spectral range. Conventional measuring arrangements for performing spectroscopy on cereal grain include a measuring cuvette for accommodating to the cereal grains and a spectrometer set up for highly sensitive transmission measurements, for example. The cuvette is exposed to light on one side. Light that passes through the cuvette due to surface reflections is detected on the opposite side. In contrast with conventional spectroscopy on liquids, the measurement results in spectroscopy on particulate samples show a sensitive dependence on the arrangement of the particles in the cuvette. Depending on the properties of the particles, such as volume, surface structure, etc., different packing densities may occur in a cuvette, thereby limiting the accuracy and reproducibility of the measurement results.
To overcome these problems, it has been proposed in the past that multiple measurements be performed on a measuring cuvette and that the measurement results be subjected to a statistical analysis. In practice, for example, it is known that the measuring cuvettes may be arranged on rotary tables that have been developed as sample holders for measurements on meat, yogurt or the like. A rotary table includes a shallow dish into which a sample of cereal grain is poured. For the spectroscopic measurement, different areas of the horizontally aligned dish are moved into the path of the beam of the spectrometer. This is accomplished by rotating the dish about a vertically aligned axis of rotation. The reproducibility of the measurement results has been improved by performing multiple measurements at different locations on a layer of particles. Nevertheless, corruption of the measurement results obtained with the horizontally adjustable rotary table has been found for the following reasons.
First, the layer in the dish does not have a uniform thickness and density. Heaps of particles are formed, causing variations in the distance from the illumination optics or the detector on the one hand and on the other hand causing the layer thickness to vary from one position to the next. In addition, certain bulk profiles develop when particles are poured in. Depending on the particle properties, the smaller or larger particles will be at the top of the dish. This results in separation and thus selection of the particles that contribute to the measurement. Finally, the available quantity of sample is frequently too small to fill a dish-like cuvette with a sufficient layer thickness. However, the extent to which the size of the cuvette can be reduced is limited because of the dimensions of the optical measurement beam and the individual particles.
Because of the low precision and reproducibility of the measurement results, spectroscopic measurements of grain have so far been of little value. For example, they are suitable for a rough classification of a grain quality, but not for a differentiated evaluation of varieties, for example, or for detecting contaminants or fusaria. There is a great deal of interest in particular in detecting even the smallest amount of impurities or fungi with a high reliability. Thus, in the case of a fusaria infestation, even if only one grain in 200 is found to have microtoxins on its surface, this is a finding which will prevent the use of this grain in a brewery. The infested cereal grain must be discovered among a multitude of grains. This presupposes in particular that the grain is positioned in the optical measurement zone so that microtoxins can be detected spectroscopically.
These problems occur not only in testing cereal grains or other agricultural products but also in general in characterizing particulate free-flowing samples or samples suspended in a liquid medium, in particular materials such as plastic granules, construction materials or the like.
The object of this invention is to provide an improved measuring arrangement, in particular for spectroscopic measurement of particulate or liquid samples with which the disadvantages of conventional measuring arrangements are overcome and which will be characterized in particular by an increased sensitivity, precision and reproducibility. Furthermore, the inventive measuring arrangement should have an expanded range of applications and should be suitable for measurement on a variety of particulate or liquid samples. The object of this invention is also to provide a method for spectroscopic measurement of particulate samples or liquid samples with which the disadvantages of the conventional methods are overcome. The improved method should permit in particular an increased sensitivity, precision and reproducibility of the measured value acquisition and should be compatible with essentially known spectroscopic measurement principles.